Phenolphthalein Colour Change Explained: From Acid to Alkali

Phenolphthalein is an organic compound (C₂₀H₁₄O₄) that appears as a white to pale yellow crystal with no smell. Its melting point is approximately 261 to 265°C, which means it is stable and easy to store in powder form.

Phenolphthalein is most widely known as an acid-base indicator, or pH indicator: a substance added to liquids to give a visual signal when they change from acidic to alkaline. Phenolphthalein as an indicator is valued for how sharp and obvious that change is.

What Causes Phenolphthalein to Change Colour?

Phenolphthalein changes colour because of a shift in its molecular structure that depends on the pH of the solution.

In acidic environments it remains colourless, because its molecular form does not absorb visible light. As the solution becomes more alkaline, typically above pH 8.3, phenolphthalein loses hydrogen ions and its structure changes. This new form absorbs visible light, giving the solution a pink to fuchsia colour.

At very high pH levels, above around 13, it may become colourless again due to further changes in its structure.

Phenolphthalein's pH Range Breakdown

Phenolphthalein's colour change is closely tied to the pH of the solution it is in. Here is how it behaves across different levels:

Acidic, below pH 7: Phenolphthalein is colourless in acid. It does not turn pink in acidic solutions, so if you are asking what colour phenolphthalein turns in acid, the answer is no colour at all.

Neutral, pH 7: The colour of phenolphthalein indicator in a neutral solution is still colourless, so it will not show in standard water.

Slightly alkaline, pH 8.3 to 10.0: In an alkaline solution, phenolphthalein indicator turns pink, deepening from pale pink to bright fuchsia as the pH increases.

Very alkaline, above pH 13: Fades back to colourless.

The phenolphthalein pH range is therefore roughly 8.3 to 10.0, which is the band where the colour change occurs. This makes phenolphthalein useful for identifying the tipping point between neutral and alkaline solutions, particularly in simple titration setups.

Discovery and Usage

Phenolphthalein was first synthesised in 1871 by the German chemist Adolf von Baeyer, who would later win the Nobel Prize in Chemistry. While investigating coal tar derivatives and the development of synthetic dyes, von Baeyer created phenolphthalein as part of his work on triphenylmethane compounds. Its vivid colour transformation in different pH environments made it an immediate candidate for use in analytical chemistry.

Since then, phenolphthalein has become one of the most widely used acid-base indicators in the world. In a phenolphthalein titration, its sharp transition from colourless to pink makes it easy to detect the moment a reaction reaches its endpoint. It is also used for testing the alkalinity of water, demonstrating pH changes in educational displays, and in some forensic chemistry methods.

Where Can I Buy Phenolphthalein in the UK?

APC Pure supplies phenolphthalein solutions and powders in a range of concentrations for laboratory, educational and general use, shipped across the UK.

Concentration should be chosen to suit the procedure being carried out. Always read the product label and safety data sheet before use.